Methods: For a six-month period routine clinical isolates collected from any patient site that resembled S. aureus morphologically at 24 hr of incubation were subcultured onto sheep blood agar, as per laboratory protocol. A 30 mg cefoxitin disk (Oxoid) was placed in the first quadrant of the subculture plate, which was incubated at 35 degrees C for 18 to 24 hours. The inhibitory zone size of each isolate was measured and categorised as susceptible (S) or resistant (R) according to current Clinical Laboratories Standards Institute (CLSI) breakpoint values (S ≥ 20 mm, R ≤ 19 mm). Comparatively, a standard cefoxitin Kirby-Bauer disk diffusion (CLSI) was performed for each S. aureus isolate. Confirmatory MRSA identification was performed using a PBP2A assay (Denka Seiken).

Results: 518 S. aureus isolates were collected from 372 patients that were evenly distributed across common specimen sites, including blood, respiratory, skin, and wounds. The direct and CLSI disk methods showed 100% breakpoint agreement (S, 79%) and cefoxitin R isolates were confirmed MRSA by PBP2A. However, the direct method provided presumptive MRSA results 24 hours sooner. Interestingly, only one isolate (0.2%) that was S by the CLSI method had an inhibitory zone between 20 and 23 mm. In contrast, using the direct cefoxitin method 72 isolates (14%) had a reduced S zone between 20 and 23 mm. Similar results were observed when the data was restricted to one isolate per patient.

Conclusion: In this study, a non-standardised cefoxitin disk test reliably predicted cefoxitin susceptibility for clinical S. aureus isolates 24 hours sooner than that provided by CLSI disk diffusion. If adopted, this would allow faster laboratory reporting and infection control interventions for presumptive MRSA-positive patients. As a precautionary measure, infection control precautions could also be considered for patients with an isolate exhibiting a reduced S inhibitory zone size (20 to 23 mm) until confirmatory testing was complete.